Diagnosing Glow Controller Problem
 

Ok, so after I replaced seal washers and o-rings in the IP valves last night the engine required quite a bit of cranking to get it going, as expected. At any rate, the glow plug light came on and during the first cranking sequence I let it cycle completely but on subsequent cranking attempts, perhaps two more 5+ second cranking attempts, I didn't wait for it to go out.

I didn't think too much about it and I drove it home and parked it. I plugged it in last night, -7F, so that it would be warm in the morning. When I went to go to work this am the glow light did not illuminate as usual even with block heater.

I tested today and found one glow plug out of spec, high ohms but not enough that it should have faulted the whole system so I checked voltage output at the controller with ignition on and found each plug was only getting 8V, not the 12V+ that they should be receiving.

I have to assume that the glow controller is toast although since I don't know what other inputs the glow controller sees (water temp?) I can't be certain.

Am I correct in that the glow controller is shot or is there something else I should check?

Thanks
Steve A

Hows the BATTERY? How's the GP circuit WIRING
 

A weak Battery does require especial Charging.

1.Proper DISTILLED water fill of each cell.
2.Nice OVERNIGHT 1.5 to 2 Amp charge with a 3 step Charger.
(May be as long as 24-48 hrs.)
3.After Full Charge cycle turn on Headlights for a 10 second period to burn off
"Surface Charge". (Or wait 2 Hrs for Battery to equalize)
4.Measure resting battery BOTH with a DMM (for Voltage)and a Hydrometer
(For Specific Gravity).The Specific Gravity Measurement is the Ne Plus Ultra,
or True Determinant.

http://www.schumacherproducts.com/applications/?id=0007

Check ALL your grounds!(and connections "Clean,Shiny and Bright")
The Pre-Glow Relay has it's own ground to Chassis.
the Engine to Chassis strap Ground must also be good.

The fact that your GP relay is delivering 8 Volts tell you it's working.
"Somezing" is interfering with delivery of Full Voltage to the GP relay.
(Or delivery of Full Voltage from the GP relay to the Glow Plugs)
[AKA WIRING from GP relay to the Gps]

Thanks for the tips, the battery is nearly new and delivering 12.9 to the relay with the ignition off.

At any rate, I've checked plug resistance, two GP's were replaced earlier this year, all plugs save for one are at about .5 ohm resistance. The other is at 18 ohm resistance but that's not bad enough to cause the system to fault completely. I'm almost certain that the relay has gone bad.

The clincher is that the relay is only allocating 8V to the gp harness connector at the relay...to every plug and hence I really think it's gone bad. That is unless there's another outside influence on the GP relay, like a temp sensor that is falsely telling the relay that glow is unnecessary.

Regards
Steve A

When these ReLays Fail...
 

It's ALMOST impossible for them to fail in a "PARTIAL" mode.
you either get Voltage @ the Pins or you do not.

You might be able to make a case for "Dirty" contactors inside the Relay,
Some have taken the cover off and gently cleaned the "Dirty" contactors
with superfine Emery.

The One GP that measures 18 Ohm is Really Bad.

When you're measuring the Voltage to the Glow Plug Harness Relay Pins ,is the
Harness plug disconnected from the ReLay?
If the Harness is still plugged into the ReLay, that 18 Ohm GP could be pulling
down the System.

What's the Voltage @ the Power supply connector (The RED ,HOT @ all times)
cable at the Glow relay when the system is in active "Glow".
That 12.9 volts with the Key in your hand is nice...BUT tells you nothing
about what's going on in operation.

I'm still hesitant to condemn your GP relay.

[connections,and grounds grounds grounds]

I agree with you Compress. The low voltage suggests that its not a straight forward "something is toast" problem. More like a bad connection. Relay contacts are just another connection. I once had an insect get in to the contacts & got fried, it gave a similar result. Supply voltage measurement under load to the GP control will reduce the possible causes list.

I'm not debating whether this plug is bad, I agree it's bad but it's not nearly as bad as some I've dealt with before on other cars. I've never had a plug that tested at this level of resistance terminate system operation.

Yes, plug disconnected and V taken directly from relay pins.

agreed, but it's not connected so the downstream components didn't impact.

You know tbh I didn't check that reading because my understanding of the ignition switch contribution to the GP relay doesn't impact voltage reading at the non-switched 12v+ connector. In other words, non-switched power, i.e. direct connection, shouldn't change.

Well I can pull it out and take it apart, I've taken them apart before, just not this variety.

Thanks for helping me try and diagnose this problem. I'll post back if I can get out there and take a reading on the 12V constant power with the ign switch on.

Steve A

Ok, the Simpsons was delayed so I had time to run and take the reading. I had to use a different meter so that introduces an element I'm not happy about but I can take another reading tomorrow with the same meter I used for the prior readings.

With the key on I read 12 to 12.1 V at the constant power lead to the relay. the reading with this meter without the key on was 12.4-12.5 so it seems inconsistent with the other meter to some degree.

Thanks again
Steve A

I do have another question that goes along with this, and that is why there's no fuse strip on this controller as there are on some others? I have to assume there's a fuse somewhere that either takes the place of the flat strip fuse or did they eliminate it completely on this model?

thanks
steve a

Wrong!
 

Your Pre-Glow ReLay has NO Strip Fuse.
(Kinda neat)
It's Electronic.
It "Senses" an "Overload" (Or a Dead Short)and disconnects until the "Problem " is fixed.

BUT,That has Nothing to do with your problem.
When the Automatic Disconnect Protection takes place,it shuts Off EVERYTHING
inside the Pre-Glow ReLay.

I've Gotta admit I do not know the Actual Voltage output of your Pre-Glow ReLay.(BUT,It ain't 12 Volts...)

For example on my W124 300D (With the same "Electronic Pre-Glow ReLay")
outputs LESS Than 11.5 Volts at the ReLay pins.

My F-1 fuse (8Amperes) powers terminal #15 on my GP Relay.
I'll bet you've a fuse like that in your fuse box.
(You Need Electrical schematic,that's specific to your Chassis/Engine.)

There are really a lot of Circuits to be tested before you can condemn the GP
Relay!

When you look at the below Testing instructions the applicable section is
Part B [Because your Pre-Glow ReLay is equipped with up to 60 seconds of
AfterGlow]
http://www.ps2cho.net/downloads/MB%20CD/W124/w124CD1/Program/Engine/602_603/15-4000.pdf

(Ignore the Big Black arrow on the Cyrillic Pictogram of your Pre-Glow ReLay
it's a marker in Another Electrical Mystery.)

[HINT: Connections,Wiring and Grounds]

Thanks for the pointer, I'll look at the testing procedure and work from that. I don't know what could have changed though but clearly something has.

Thanks
Steve A

Well your intuition was better than mine, and I thank you for it! It ended up being the contacts, which didn't look bad at all but after cleaning with a very fine needle file my voltage jumped up to just under 12V at the pins, ignition switch on. I hesitate to say exactly what voltage was since I've used two different cheapy meters and they were off by about .5V (one said 11.9 and the other said 12.4), but at any rate, I have the glow circuit back and in good working order.

I will however, order up four new GP's for the ones that weren't replaced last January in the event that they do go bad, and to cover the one that's clearly on its way out (18ohm resistance). No one in town (save for the dealer I suspect) stocks plugs on the shelf for this car.

Thanks to all who contributed to my saving $100 this week...woo hoo!

Anyway, on to the next thing to be fixed. I have front wheel grease seals in mid-week for wheel bearing repack and also a idler arm bushing kit to install and tie rods to replace. Hopefully then I can get this thing aligned! Of course that's not the end of the work....

steve a

Excellent !
 

"Somedays, the only reason to own a Mercedes is to stay busy in the Garage."

Couldn't agree more, only problem is that there's already two other diesel projects in the garage at the moment, and neither one could come out to make room, so this repair had to take place in the driveway. Fortunately it was about 17F outside so it wasn't horrible, and for the relay repair I was inside.

steve a

Ok guys, this problem continues to plague me. I replaced the one GP that was testing at 18 ohms so that eliminates that plug as a problem, but even before that and after the replacement I continue to have controller failure randomly.

I can pull the controller and clean the contacts and it'll work again fine, perhaps for several days, but yesterday it happened again and I cleaned them, controller worked and I got home shut the car down and went out today and no glow...so it essentially worked once. Most of the time I get a dozen glow cycles out of the thing before it shuts down...clean, reinstall and then it works. But this has to end, I cannot always be fighting this thing and I cannot believe that it's this unreliable.

Anything else on this I should look at before breaking down and buying a new one?

Thanks
Steve A

IF there's nothing wrong with your engine Wiring harness...
 

EDIT: Just so all of this won't be "All Fun,All the Time"...
Physically locate the separate "Ground" wire for the Electronic Pre-Glow Relay
at it's terminus on the body.
And assure yourself the Connections and the Cable are good.
An intermittent Grounding Connection sounds awfully like your symptoms, TOO
(I admit I'm grabbing at non-existent Life Rings, here.)
BUT, maybe, each time you R+R the Relay to clean the contacts / you also
"Fix" a Ground problem,Temporarily.

1992 is awfully close to the "Middle Nineties"
"Automatic Self (Ecologically) Degrading Engine Wiring Harnesses"
Mercedes bought from a Vendor and "Gifted" [Spelled, F**ked] some owners with.

(Recycling was all the rage in Deutschland, and the Vendor screwed up on the insulation specifications. OR MAYBE NOT?!?)

I'd clean the contactor(s) once more and make sure they're super duper SHINY and Smooth/Mirror Like and then apply Stabilant22 to both sides.
(It's about $30.00 for 5Ml which makes 15Ml solution)

[OUTRAGEOUS! But the stuff works.]

And it's cheaper than a New GP Relay.


http://www.stabilant.com/techt02h.htm
http://www.stabilant.com/sizes01h.htm

Example of pricing:http://www.posthorn.com/Stab_2.html